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        <td class="header">&nbsp; Ellipsoid Correction</td>
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<h3>Ellipsoid Correction GG Operator</h3>

<p>The operator implements the Geolocation-Grid (GG) method [1]. The implementation is exactly the same as for the&nbsp;<a href="RangeDopplerGeocodingOp.html">Range Doppler Terrain Correction operator</a>
    except that the slant range is computed from slant range time tie point of the source product&nbsp;instead of using
    DEM.</p><h4>Major Processing Steps</h4>

<p>
    Some major steps of the algorithm are given below:
</p>
<ol>
    <li> Get the latitudes and longitudes for the four corners of the source image;</li>
    <li> Determine target image boundaries based on the scene corner latitudes and longitude;</li>
    <li> Get range and azimuth pixel spacings from the metadata of the source image;</li>
    <li> Compute target image traversal intervals based on the source image pixel spacing;</li>
    <li> Compute target image dimension;</li>
    <li> Get tie points (latitude, longitude and slant range time) from geolocation LADS of the source image;</li>
    <li> Repeat the following steps for each cell in the target image raster:</li>
    <ol type="a">
        <li> Get latitude and longitude for current cell;</li>
        <li> Determine the corresponding position of current cell in the source image and
            the 4 pixels that are immediately adjacent to it;
        </li>
        <li> Compute slant range R for the cell using slant range time and bi-quadratic interpolation;</li>
        <li> Compute zero Doppler time T for the cell;</li>
        <li> Compute bias-corrected zero Doppler time Tc = T + R*2/C, where C is the light speed;</li>
        <li> Compute azimuth index Ia using zero Doppler time Tc;</li>
        <li> Compute range image index Ir using slant range R;</li>
        <li> Compute pixel value x(Ia,Ir) using bi-linear interpolation and set it for current sample in target image.
        </li>
    </ol>
</ol><h4>Parameters Used</h4>

<p>
    The following parameters are used by the operator:
</p>
<ol>
    <li>Source Band: All bands (real or virtual) of the source product. User can select
        one or more bands. For complex product, i and q bands must be selected together. If
        no bands are selected, then by default all bands are selected.
    </li>
    <li value="2">Image resampling method: Interpolation methods for
        obtaining pixel values from
        source image.
    </li>
    <li>Map Projection: The map projection types. The orthorectified image will&nbsp;be presented with the user selected
        map projection.
    </li>
</ol>
<ol>
</ol>
<p>The output will be expressed in WGS84 latlong geographic
    coodinate.</p>

<p><img style="width: 500px; height: 500px;" alt="" src="images/ellipsoid_correction_gg_dlg.jpg"><br>
</p>

<p>[1] Small D., Schubert A., Guide to
    ASAR Geocoding, Issue 1.0, 19.03.2008</p>
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